1. Field of the Invention
This invention generally relates to automatic gain control (AGC) circuits and, more particularly, to a system and method for compensating the error in voltage values used in biasing an advanced mobile phone service (AMPS) wireless communications device transmitter.
2. Description of the Related Art
The output signal from transmitter circuitry, such as a power amplifier used to transmit signals in a wireless communications device, is inherently sensitive to changes in temperature, operating frequency and lot variations. As a result, it is well known to employ an AGC circuit to adjust the transmitter output level for more accurate output. In an AGC circuit, typically the transmitter output level is sampled and converted into a dc voltage, with the dc voltage used to set a feedback signal to adjust the output level from a power amplifier.
FIG. 1 is a schematic block diagram of a conventional transmitter with an AGC loop (prior art). An RF signal is received and amplified by transmitter circuitry, with the amplified signal used to drive an antenna. The amplified signal is also measured by a measuring circuit to provide a transmitter output measurement. The transmitter output measurement is fed into an operational amplifier (op amp), which is typically part of a loop filter (not shown). The transmitter output measurement is compared to a reference voltage level that represents the desired transmitter output measurement value. Responsive to the comparison, the op amp generates an adjusted transmit bias control value, which is then fed into the transmitter as an amplifier bias. The op amp is used to adjust the amplifier bias voltage until the transmitter output measurement voltage is substantially equal to the desired reference voltage.
It may be desirable to enable the transmitter to operate at a plurality of output levels. For example, the transmitter may be set to have a gain of 0, 3, 6, or 12 decibels (dB). At each predefined output level the AGC will typically use a corresponding reference voltage to compare against the measured transmitter output voltage. Alternately, the transmitter output measurement can be attenuated or amplified to correspond with the desired transmitter output for each predefined level. Although the AGC loop is shown as an analog circuit, it is also well known to convert the voltage levels to binary values and to operate the loop as a digital circuit. It should be understood that many variations of AGC loops exist, but the principle of stabilizing upon a reference is common to all the variations.
The process of stabilizing the transmitter output with an AGC loop takes time. The time delay in stabilizing the output is dependent on several factors, including the initial amplification error and how accurately the output level must be set. The AMPS specification requires that the transmitter output level settle to a specified power level within 20 milliseconds. To speed signal stabilization, it is well known to provide an initial transmit bias control value, which is an estimated initial transmitter output measurement value. An initial transmit bias control value may be set for each desired transmit output level. Accordingly, each time the transmitter is set to operate at a different output level, the initial transmit bias control value associated with that output level is used to facilitate timely stabilization.
Typically, transmit bias control values are determined and set using a calibration procedure. In this regard, each transmitter may be tested and assigned control values, or transmit bias control values may be set for a lot of transmitters. The bias control value(s) assigned to the transmitters are then typically stored in a non-volatile memory. In practice it is desirable to perform calibration in an environment that approximates the typical operating conditions of the transmitter. In this way the bias control values will better reflect the initial adjustment needed to stabilize the transmitter output signal.
As the AGC circuit operates to stabilize the transmitter output signal at a predefined level, the initial transmit bias control value is switched out of the circuit. Alternately, the op amp may be designed to provide higher gain for the actual transmitter output measurement, essentially ignoring the initial transmit bias control value when the op amp input values are close.
However, the initial transmit bias control values may be subject to substantial inaccuracies, reducing the effectiveness of the initial transmit bias control value and increasing the time delay in stabilizing the transmitter output signal. For example, variations between transistor lots, ambient operating temperature, battery voltage, transistor junction temperature, and the heatsink (wireless device circuit board) temperature lead to inaccuracies in the supplied transmit bias initial values. Since the size of wireless devices, especially mobile telephones, continues to shrink, the inaccuracies introduced as a result of heatsink, and ultimately junction temperature continue to be a problem. Inaccurate initial transmit bias values or changing environmental conditions limit the ability of an AMPS mode wireless telephone to operate within specifications.
It would be advantageous if a wireless communications device operating in AMPS mode could acquire the steady state transmitter output level quickly even in changing environmental conditions.